Heater element



March 31, 1970 R, CLEMENT ET AL 3,504,327

HEATER ELEMENT 2 Sheets-Sheet 1 Driginal Filed June 13, 1966 Nimh 31, 19.70 R, @EMNT TAL. 3,504,327v

HEATER ELEMENT Original Filed June 13. 1966 2 Sheets-Sheet B United States Patent O 3,504,327 HEATER ELEMENT Ren Clement and Maurice Ducos, Paris, France, assignors to Societe de Fabrication dElements Catalytiques, a corporation of France Continuation of application Ser. No. 557,269, June 13, 1966. This application Nov. 1, 1968, Ser. No. 774,576 Claims priority, application1 France, June 14, 1965,

U.S. Cl. 338-262 8 Claims VABSTRACT F THE DISCLOSURE An electrical heater element. This element comprises an electrically conducting core. A purality of superimposed layers cover said cores; the layers are made alternately of refractory compounds and of a metal capable, after melting, of wetting the two adjacent oxide layers. Preferably, the metal has a melting temperature lower than the working temperature of the heater.

This is a continuation application of the application Ser. No. 557,269, tiled June 13, 1966, now abandoned.

The present invention relates to heater elements more particularly of the type used, for example, in electrical ovens. Generally, such heater resistors consist either of refractory metals, or of intermetallic compounds, or of carbides, the substance generally used being silicon carbide.v

The resistors, which are generally in the shape of rods or cylinders, are prepared from powder by moulding and by extrusion with a plastic binder. The rough parts are then increased in density by high temperature heat treatment, often in a protecting atmosphere (argon, nitrogen, hydrogen) to prevent the grains from being oxidized.

This production process is costly and many parts have to be scrapped, owing to distortion during the baking process or to the formation of cracks which'are the origin of hot points which quickly deteriorate the resistors. Last, but not least, the temperature of utilisation must not exceed l450 C.

It is an object of the present invention to provide an electrical heater element free of these drawbacks.

According to the invention, there is provided an electrical heater element comprising an electrically conducting core, a plurality of successive layers covering said core, said layers being alternately of refractory compounds and of a metal capable of wetting the adjacent oxide layer.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings accompanying the following descriptionl and in which:

iFIG. 1 shows in longitudinal section one example of a heater element according to the invention;

FIG. 2 shows a transverse section of the element in its coated parti and FIG. 3 shows a modification in longitudinal section.

FIG. l shows a cylindrical graphite core 1, with a reduced diameter in the central region thereof. The core is coated, except for its ends, which are clear for the purpose of making electric connections, with a deposit consisting of alternate layers of refractory material 2 and of metal 3 whose melting temperature may be less than the working temperature of the heater. A metal layer 4 formed at the ends of the element ensures good electric contact between the graphite core and the electric connection collars.

FIG. 2 shows a transverse section of the element in its coated part. It shows the successive concentric deposits of refractory oxide 2 and of metal 3 on the graphite core.

The refractory layers consist of oxides or oxide compounds, with a melting point higher than l600 C; for example, aluminium oxide, zirconium oxide (stabilized with lime or yttrium oxide), chromium oxide, spinel of magnesia (AlzOg-MgO), zirconium silicate, etc. may be used.

The iirst refractory layer need not be formed of a single oxide layer but of several superposed thin layers of different oxides With expansion coecients of increasing value to form an expansion gradient between the graphite core which is subject to little expansion and the metal layer which, in general, has an expansion coetlicient greater than 15 0X 10-7.

The nature of the metal has to be carefully chosen, for its vapour pressure, at the working temperature of the heater element must not exceed 10 millimetres of mercury, since a high vapour pressure would result in the destruction of the outer oxide layer.

The metal may be chosen, for example, from among the following: iron, nickel, cobalt, copper, chromium, silicon, titanium or their alloys.

Further, the metal or alloy has to be chosen for its capacity of wetting the neighbouring oxide layers; it has been found that alloys containing a certain amount of titanium, such as, for example, nickel-titanium or copper-titanium, wet alumina layers perfectly. This characteristic allows the metal layers to spread out well between the oxide layers in a continuous and tight deposit. If the metal fails-to wet adjacent oxide deposits, it runs together, after melting, into small islands which leave large pores through which oxygen from the air can penetrate to oxidize the graphite core.

It is to be understood that the number of layers is not limited and that, to secure a better seal, three or more layers of oxide or of refractory compounds may be used.

To make the metal layer more compact, it may be cornpressed by elastic membranes in isostatic compression.

Some numerical data will be given hereinafter by way of a non-restrictive example to provide a better understanding of the invention.

The graphite bar has a diameter of about 2O mm. and is 250 mm. long. It is reduced in its middle part over a length of about 8O mm. to a diameter of 3 mm. This operation can be easily performed, for example on a lathe, by choosing a fine grain, easily machined graphite.

Using a spraying gun, a layer of alumina is deposited on the reduced portion, the layer thus formed extending over some 10 mm. on the larger diameter ends. This alumina layer is then sprayed with a layer of copper (66%)-titanium (33%) alloy, care being taken that this metal deposit should not extend beyond the alumina layer underneath, for the metal would short-circuit the graphite.

A third layer of alumina is then deposited to cover the first two layers completely.

At each end of the heater element, a silver-copper a1- loy is sprayed over a length of about l0 to 15 mm. on the graphite to ensure electrical contacts.

If a suiliciently large current is applied to the rod, the middle part very quickly glows red. For a dissipated power of about 500 watts, the temperature reaches 1500" C. This temprature can be safely sustained or even exceeded up to 1700 C.

It is to be understood that the shape of the bar is immaterial. It may be, for example, prismatic or parallelipedic.

The applicant has also found that the heating element according to the invention is substantially improved, as to the admissible working temperature and the useful life thereof, if an additional layer of an oxidizable substance, for example a layer of molybdenum, tungsten,

niobium or tantalum is provided between the graphite core 1 and the refractory material 2.

The applicant is inclined to believe, although of course this explanation does not bind him, that this intermediate layer has probably the effect of compensating the difference between the thermal expansion coefficients of the graphiteand of the layers superimposed thereupon, this being due to the fact that this intermediate layer is so selected that its expansion coefficient has a value which is comprised between that of the graphite expansion coeicient and that of the oxide layer expansion coefficient. Also a layer of carbide is thus formed.

As indicated above, a greater number of superimposed layers may be applied.

In the same manner as in the embodiment of FIG. 2, a silver-copper alloy covers the end portions of the graphite core for providing electric connections.

When the current flowing through the graphite core increases, the core reddens rapidly. For a dissipated power of about 1 kw. the temperature reaches l700 C., which temperature does not impair the graphite core.

Another effect of the intermediate layer is that it traps air oxygen which might have penetrated between the protective layers.

The applicant has also found that the metal layer or layers interposed between the oxide layers may be advantageously so selected so as to reduce supercially at high temperature the oxide layers. Thus, aluminium layers may be used to separate alumina layers which they reduce in part and on which aluminium is perfectly spread out.

FIG. 3 shows by way of example a bar 10 of graphite 280 mm. long and having a 20 mm. diameter. The graphite core has its diameter reduced to 3 mm. in its intermediate portion over some 80 mm.

By means of a pistol, a molybdenum coating 11 is projected on the core 1, this coating extending up to some mm. from the ends of the core. An aluminium layer 12 is formed on the molybdenum layer. Then, an alumina layer 13 is formed on the aluminium layer 12 and a further aluminium layer 14 is formed on layer 13.

Of course, the invention is not limited to the embodiments described and shown which were given only by way of example.

What is claimed is:

1. An electrical heater comprising:

an electrically conducting core,

a plurality of successive layers covering said core, said layers being alternately of refractory and metal compounds capable of wetting the adjacent refractory layer, and

.4 a further metal layer interposed between said core and said successive layers, said further layer being of an oxidisable metal having an expansion coefficient comprised between the expansion coeicients of said core, and the layer adjacent thereto, and a melting point higher than 2500" C. 2. A heater as claimed in claim 1, wherein said element comprises a cylindrical core, having a restricted center portion covered with said layers, and two metallized end portions.

3. A heater as claimed in claim 1, wherein said core is of graphite.

. a plurality of successive layers covering said core, said layers being alternately of refractory and metal selected from the group comprising iron, nickel, cobalt, copper, chromium, silicon, titanium and aluminium, wherein a further metal layer lis interposed between said core and said successive layers, said further'layer being of an oxidisable metal having an expansion coecient comprised between the expansion coeicients of said core andthe layer adjacent thereto, and a melting point higher than 2500 C.

6. A heater as claimed in claim 1, wherein said further metal layer is capable of reducing said refractory layers.

7. A heater as claimed in claim 5, wherein said metal has a vapour pressure at the working temperature lower than 10 millimeters of mercury.

8. A heater as claimed in claim 6, wherein said metal compound is an alloy.

References Cited UNITED STATES PATENTS 1,200,352 10/1916 Hadaway 338-243 X 1,736,745 11/1929 Lohmann 338-256 X 1,788,146 1/1931 Boyles 338-'275 X 2,385,702 l 9/1945 Hediger 338`275 X 3,348,929 10/ 1967 Valtschev.

ELLIOT'A. GOLDBERG, Primary Examiner U.S. Cl. X.R. 

